In an antenna for a TV (television) broadcasting particularly for a terrestrial digital TV broadcasting (470-710 MHz) formed on a rear glass window glass panel, characteristics for improving the picture quality of a TV are required by controlling the directivity toward a desired wave to suppress the interference with an undesired wave coming from the direction other than toward the desired wave. In particular, a reception performance is reduced due to the Doppler shift when a motor vehicle runs at a high speed, so that the sensitivity difference between the desired and undesired waves, i.e. an F/B (front/back) ratio, is required by 10 dB or more.
For this requirement, it has been known in Japanese Patent Publication No. 2002-135025, for example, that a directional antenna comprising a reflector and director may be effectively used. It has also been known in Japanese Patent Publication No. 2003-283405 that the directivity may be realized by functioning the roof (metal) of a motor vehicle as a reflector.
In the case that a conventional antenna is provided on a rear window glass panel having heating lines, various problems are caused. For example, the controlling of the directivity toward a desired wave becomes difficult due to the effect of the heating lines, resulting in the difficulty in obtaining a desired F/B ratio. This is due to the fact that the directivity of an antenna on a rear window glass panel is varied in a vertical direction with respect to the surface of a rear window glass panel as shown in FIG. 1 by the dotted-line 2 to decrease the sensitivity of the antenna in a horizontal direction. The reference numeral 4 shows the directivity of the antenna in the case that there are no heating lines on the panel.
Even if a reflector and director which are both parasitic elements are used, the control of the directivity toward a desired wave is difficult due to the effect of the heating lines.